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+# Entity Integrity
+**Entity integrity** is the gurantee provided by the database of a 1:1 correspondence between real-world entities and their digital representations.
+As such, it is a prerequite for all other aspects of data integrity.
+
+For entity integrity, each real-world entity must be represented by exactly one unique record in the database; conversely, each record must correspond to a single, distinct real-world entity.
+
+Imagine what kinds of difficulties would arise if entity integrity broke down in the systems you interact with every day.
+For example, what would happen if your university or HR department had two different identifies for you in their records?
+What would happen if they occasionally updated your records with another person's information?  
+What if the same ocurred in your dentist's office?
+
+Without entity integrity, it is impossible to maintain other aspects of integrity within the database.
+For example, a foreign key relationship assumes that every referenced entity exists uniquely and correctly in the database—an assumption that can only hold true if entity integrity is enforced.
+
+# Challenges to Entity Integrity
+The challenge of ensuring entity integrity lies in the fact that it cannot be fully solved by the database system alone.
+A reliable system for identifying objects in the real world must be established outside the database to ensure that each entity has a unique and persistent identifier that can be consistently used across all related data records by all participants.
+
+This requires setting up a disciplined process outside the database.
+
+For example, establishing the Social Security system in the United States required a reliable identification of workers by all employers to report their income across their entire careers.
+For this purpose, in 1936, the Federal Government established a new process to ensure that each US worker would be assigned a unique number, the Social Security Number (SSN).
+The SSN would be assigned at birth or at entering the country for employment and no person would be allowed to have two such numbers. 
+Establishing and enforcing such a system is not easy and takes a considerable effort.
+The SSN allows for the accurate and consistent representation of individuals across various government databases, ensuring that each person is correctly identified. 
+
+**Question**: Why didn't the US government need to assign unique identifiers to tax payers when it began levying federal taxes in 1913?
+
+**Question**: What abuses would become possible if a person could obtain two SSNs or if two persons could share the same SSN?
+
+**Learn more** about the history and uses of the SSN:
+  * [History of establishing SSN.](https://www.ssa.gov/history/ssn/firstcard.html)
+  * [How the SSN works.](https://www.ssa.gov/policy/docs/ssb/v69n2/v69n2p55.html)
+  * [IRS timeline.](https://www.irs.gov/irs-history-timeline)
+
+Similar rigor is required for identifying other objects in the real world:
+* VIN for all vehicles
+* Animal identification number for pets and domestic animals
+* Registration numbers for aircraft
+
+In summary, designing a database may need to begin with designing good identification processes in the enterprise that it supports.
+
+When a neuroscience lab sets out to establish a data management process, the first thing it needs to do is to establish a uniform system to identify test subjects, experiments, and protocols.
+
+# Entity Integrity in Schema Design
+
+Several key aspects of relational database design contribute to maintaining entity integrity:
+
+1. **Clear Table Definitions Reflecting Entity Types**
+
+Each table in a relational database should clearly indicate the type of real-world entity it represents.
+The table name plays a crucial role in conveying this information.
+For instance, if a table is named `Person`, the database must enforce entity integrity for individuals, ensuring each record corresponds to a unique person.
+
+However, if the table uses identifiers that do not ensure a 1:1 mapping to actual persons—such as cell phone numbers, which might be shared or changed—a more appropriate table name should be chosen, like `UserAccount`, to reflect the specific entity type being represented.
+This clarity helps avoid confusion and ensures that the database design accurately mirrors the real-world relationships it models.
+
+2. **Primary Keys and Unique Indexes**
+
+Every table in a relational database must have a primary key, and the primary key attributes cannot be nullable. This requirement is crucial for maintaining entity integrity, as it prevents the creation of records that cannot be uniquely identified.
+The table may haveother unique indexes that enforce a 1:1 correspondence between records and the real-world entities they represent.
+Secondary unique indexes can enforce uniqueness on other attributes that need to be unique across the table but may be nullable.
+For example, for the `Person` table, unique indexes can be enforced on email addresses, usernames, drivers licenses, cellphone numebrs, or social security numbers.
+These indexes ensure no duplicate entries for attributes intended to be unique, further supporting entity integrity.
+
+Depending on the business needs, entity integrity may be loosened.
+One business may tolerate multiple digital identities per person while another may allow multiple persons sharing a digital identities. 
+Other business would require strict 1:1 match.
+These requirements would lead to different approaches to managing identities and enforcing entity integrity.
+
+# Practical Examples of Ensuring Entity Integrity
+Consider the importance and challenges of entity integrity in the following scenarios.
+Think what processes each organization needs to implement to establish and verify unique identification.
+What unique indexes does the database need to enforce?
+
+- **Students at a University:** Each student is uniquely identified by a student ID number assigned during enrollment, ensuring entity integrity by preventing duplicate student records
+- **Kids at a Daycare Center:** A unique enrollment number or child ID can be assigned upon registration, ensuring consistent and unique representation of each child.
+- **Airline Passengers:** Passengers can be uniquely identified by a booking reference number or frequent flyer number, ensuring accurate linkage of travel details to individual records.
+- **Gym Members:** A unique membership ID assigned at joining ensures that each member’s activities are accurately recorded without duplication.
+- **Online Video Game Players:** Each player is assigned a unique gamer tag or player ID, ensuring consistent and unique tracking of player interactions and achievements.
+- **Posts on Facebook:** Each post is associated with a unique post ID, linking the content to the specific user and ensuring that each post is uniquely identified.
+- **Mortgage Loans:** Each mortgage loan is identified by a unique loan number or mortgage ID, ensuring that all related transactions and documents are consistently linked.
+
+# Summary
+Entity integrity is a critical aspect of relational database design that ensures the accurate and consistent representation of real-world entities within a database. 
+By implementing clear table definitions, enforcing primary keys and unique indexes, and ensuring mandatory primary keys, you can create robust databases that faithfully mirror the real-world entities and relationships they are intended to manage. 
+This foundation of entity integrity supports reliable and accurate data management, enabling databases to function effectively in representing and maintaining the integrity of the data they store.